Catalyst for exhaust gas purification

ABSTRACT

To provide an exhaust gas purifying catalyst which can maintain the catalytic activity of Pt at a high level over a long time and can achieve satisfactory emission control performance, an exhaust gas purifying catalyst is prepared so as to contain a composite oxide having a perovskite structure represented by the general formula (1):
 
A 1-x A′ x B 1-y-z B′ y Pt z O 3   (1)
wherein A represents at least one element selected from rare-earth elements and essentially including one or more rare-earth elements each having a valence of 3 as the only valence; A′ represents at least one element selected from alkaline earth metals and Ag; B represents at least one element selected from Fe, Mn, and Al; B′ represents at least one element selected from transition elements excluding Pt, Fe, Mn, Co, and the rare-earth elements; and x, y, and z are atomic ratios satisfying the following relations: 0&lt;x≦0.5, 0≦y&lt;0.5, and 0&lt;z≦0.5.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying catalyst whichefficiently purifies carbon monoxide (CO), hydrocarbons (HC), andnitrogen oxides (NOx) contained in emissions (exhaust gases) typicallyfrom automobile engines.

BACKGROUND ART

Noble metals such as Pt (platinum), Rh (rhodium), and Pd (palladium)have been widely used as catalytic components of three-way catalystswhich can simultaneously clean up carbon monoxide (CO), hydrocarbons(HC), and nitrogen oxides (NOx) contained in emissions.

Among these noble metals, Pt satisfactorily oxidizes CO even at lowtemperatures but exhibits insufficient thermostability. For improvingthe thermostability, Pt is supported by a composite oxide, for example,having a perovskite structure represented by a general formula: ABO₃ byimpregnation. In addition, by incorporating Pt into a composite oxide asits constitutional component, the thermostability can further beimproved and emission control performance can be increased than in thecase where Pt is supported by such a composite oxide.

Proposed examples of such composite oxides each having a perovskitestructure and containing Pt as a constituent areLa_(0.4)Sr_(0.6)Co_(0.95)Pt_(0.05)O₃ (Japanese Laid-open (Unexamined)Patent Publication No. Hei 5-76762);La_(0.9)Ce_(0.1)Co_(0.98)Pt_(0.02)O₃, La_(1.0)Co_(0.9)Pt_(0.1)O₃, andLa_(1.0)Co_(0.8)Pt_(0.2)O₃ (Japanese Laid-open (Unexamined) PatentPublication No. Hei 6-100319); La_(0.8)Sr_(0.2)Cr_(0.95)Pt_(0.05)O₃,La_(1.0)Ni_(0.98)Pt_(0.02)O₃, La_(1.0)Co_(0.9)Pt_(0.1)O₃,La_(1.0)Fe_(0.8)Pt_(0.2)O₃, and La_(0.8)Sr_(0.2)Cr_(0.95)Pt_(0.05)O₃(Japanese Laid-open (Unexamined) Patent Publication No. Hei 6-304449);and La_(1.0)Co_(0.9)Pt_(0.1)O₃, La_(1.0)Fe_(0.8)Pt_(0.2)O₃,La_(1.0)Mn_(0.98)Pt_(0.02)O₃, La_(0.8)Sr_(0.2)Cr_(0.95)Pt_(0.05)O₃,La_(1.0)Co_(0.95)Pt_(0.05)O₃, and La_(1.0)Mn_(0.98)Pt_(0.02)O₃ (JapaneseLaid-open (Unexamined) Patent Publication No. Hei 7-116519),La_(0.2)Ba_(0.7)Y_(0.1)Cu_(0.48)Cr_(0.48)Pt_(0.04)O₃ andLa_(0.9)Ce_(0.1)Co_(0.9)Pt_(0.05)Ru_(0.05)O₃ (Japanese Laid-open(Unexamined) Patent Publication No. Hei 8-217461).

When the above-mentioned composite oxide comprises a rare-earth elementalone on the A site and Cr (chromium), Ni (nickel), and/or Cu (copper)alone as a transition element in addition to Pt on the B site of theperovskite structure represented by the general formula: ABO₃, Ptbecomes unstable in the perovskite structure under oxidative-reducingatmospheres, its grains grow after long-term use and the resultingcatalyst may exhibit remarkably reduced catalytic activity.

DISCLOSURE OF INVENTION

Accordingly, an object of the present invention is to provide an exhaustgas purifying catalyst which can maintain the catalytic activity of Ptat a high level over a long time and can achieve satisfactory emissioncontrol performance.

The present invention provides an exhaust gas purifying catalystcontaining a composite oxide having a perovskite structure representedby the general formula (1):A_(1-x)A′_(x)B_(1-y-z)B′_(y)Pt_(z)O₃  (1)wherein A represents at least one element selected from rare-earthelements which essentially include one or more rare-earth elements eachhaving a valence of 3 as the only valence; A′ represents at least oneelement selected from alkaline earth metals and Ag; B represents atleast one element selected from Fe, Mn, and Al; B′ represents at leastone element selected from transition elements excluding Pt, Fe, Mn, Co,and the rare-earth elements; x is an atomic ratio satisfying thefollowing relation: 0<x≦0.5; y is an atomic ratio satisfying thefollowing relation: 0≦y<0.5; and z is an atomic ratio satisfying thefollowing relation: 0<z≦0.5.

It is preferred that A represents at least one element selected from La,Nd, and Y; A′ represents at least one element selected from Mg, Ca, Sr,Ba, and Ag; and B′ represents at least one element selected from Rh andRu in the general formula (1).

In the general formula (1), y and z preferably satisfy the followingrelation: 0<y+z≦0.5.

Preferably, x and z in the general formula (1) satisfy the followingrelation: x=z, provided that x and z satisfy the following relation:2x=z when A′ is Ag.

BEST MODE FOR CARRYING OUT THE INVENTION

The exhaust gas purifying catalyst of the present invention is, thegeneral formula (1)

To achieve the above object, the exhaust gas purifying catalyst of thepresent invention comprises a composite oxide having a perovskitestructure represented by the general formula (1):A_(1-x)A′_(x)B_(1-y-z)B′_(y)Pt_(z)O₃  (1)wherein A represents at least one element selected from rare-earthelements and essentially including one or more rare-earth elements eachhaving a valence of 3 as the only valence; A′ represents at least oneelement selected from alkaline earth metals and Ag; B represents atleast one element selected from Fe, Mn, and Al; B′ represents at leastone element selected from transition elements excluding Pt, Fe, Mn, Co,and the rare-earth elements; x is an atomic ratio satisfying thefollowing relation: 0<x≦0.5; y is an atomic ratio satisfying thefollowing relation: 0<y≦0.5; and z is an atomic ratio satisfying thefollowing relation: 0<z≦0.5.

More specifically, the composite oxide has a perovskite structure andessentially comprises, on the A site, at least one element which isrepresented by A, is selected from rare-earth elements and essentiallyincludes one or more rare-earth elements each having a valence of 3 asthe only valence, and at least one element which is represented by A′and is selected from alkaline earth metals and Ag. Pt can be stablycontained in the perovskite structure by allowing the A site toessentially comprise at least one element which is represented by A, isselected from rare-earth elements and essentially includes one or morerare-earth elements each having a valence of 3 as the only valence, andat least one element which is represented by A′ and is selected fromalkaline earth metals and Ag.

In addition, the composite oxide, on the B site, essentially comprisesat least one element which is represented by B and is selected from Fe,Mn, and Al, optionally comprises at least one element which isrepresented by B′ and selected from transition elements excluding Pt,Fe, Mn, Co, and the rare-earth elements, and essentially comprises Pt.Pt can further be stably contained in the perovskite structure byallowing the B site to essentially comprise Pt and at least one elementrepresented by B selected from Fe, Mn, and Al.

The rare-earth element represented by A on the A site essentiallycomprises one or more rare-earth elements each having a valence of 3 asthe only valence. The “rare-earth element having a valence of 3 as theonly valence” is a rare-earth element always having a valence of 3.Examples thereof include Sc (scandium), Y (yttrium), La (lanthanum), Nd(neodymium), Pm (promethium), Gd (gadolinium), Dy (dysprosium), Ho(holmium), Er (erbium), and Lu (lutetium).

La, Nd, and Y are preferred as these rare-earth elements each having avalence of 3 as the only valence. The use of La, Nd, and/or Y furtherstabilizes the perovskite structure.

The rare-earth element represented by A may further comprise one or morerare-earth elements each having a variable valence of 3 or 4, such as Ce(cerium), Pr (praseodymium), and Tb (terbium) and/or a rare-earthelement having a variable valence of 2 or 3, such as Sm (samarium), Eu(europium), Tm (thulium), and Yb (ytterbium). In this case, the atomicratio of the rare-earth element having a valence of 3 as the onlyvalence is preferably 0.5 or more. If the atomic ratio of the rare-earthelement having a valence of 3 as the only valence on the A site is lessthan 0.5, Pt may not be stabilized in the perovskite structure.

These rare-earth elements can be used alone or in combination.

Examples of the alkaline earth metal represented by A′ on the A siteinclude Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba(barium), and Ra (radium). These alkaline earth metals can be used aloneor in combination.

Examples of the alkaline earth metal and/or Ag represented by A′ includealkaline earth metals such as Be, Mg, Ca, Sr, Ba, and Ra, and/or Ag.Preferred examples thereof are Mg, Ca, Sr, Ba, and/or Ag. The use of Mg,Ca, Sr, Ba, and/or Ag further stabilizes the perovskite structure.

On the A site, x is an atomic ratio satisfying the following relation:0<x≦0.5. Namely, the alkaline earth metal and/or Ag represented by A′,alone or in combination in arbitrary proportions, is contained on the Asite in the total atomic ratio of A′ of 0.5 or less, and preferably 0.2or less. The rare-earth element(s) represented by A, alone or incombination in arbitrary proportions, is contained on the A site in sucha total atomic ratio of A as to be the balance of the alkaline earthmetal and/or Ag represented by A′. If the atomic ratio of the alkalineearth metal and/or Ag represented by A′ exceeds 0.5, Pt may not bestabilized in the perovskite structure.

Each of Fe (iron), Mn (manganese), and Al (aluminum) which arerepresented by B and essentially contained together with Pt on the Bsite can be used alone or in combination. The use of Fe, Mn, and/or Alfurther stabilizes the perovskite structure under a reducing atmosphere.The use of Fe reduces environmental loads and improves the safety.

The transition elements represented by B′ excluding Pt, Fe, Mn, Co, andthe rare-earth elements is arbitrarily contained on the B site with Pt.Such transition elements are not specifically limited and includeelements having atomic numbers of 22 (Ti) through 30 (Zn), atomicnumbers of 40 (Zr) through 48 (Cd), and atomic numbers of 72 (Hf)through 80 (Hg) in the Periodic Table of Elements (IUPAC, 1990), exceptfor Pt, Fe, Mn, and Co. Specific examples thereof include Cr (chromium),Ni (nickel), Cu (copper), Rh (rhodium), and Ru (ruthenium), of which Rhand Ru are preferred. The use of Rh and/or Ru improves the activity atlow temperatures. These transition elements can be used alone or incombination.

On the B site, y is an atomic ratio satisfying the following relation:0≦y<0.5. Namely, each of the transition elements represented by B′ andexcluding Pt, Fe, Mn, Co, and the rare-earth elements, alone or incombination in an arbitrary atomic ratio, is contained on the B site inan total atomic ratio of B′ of less than 0.5, and preferably less than0.4. If the atomic ratio of the transition elements excluding Pt, Fe,Mn, Co, and the rare-earth elements is 0.5 or more, Pt may not bestabilized in the perovskite structure. When the transition elementrepresented by B′ excluding Pt, Fe, Mn, Co, and the rare-earth elementsis Rh and/or Ru, the cost may not be reduced.

The atomic ratio z satisfies the relation: 0<z≦0.5. Namely, Pt iscontained on the B site in an atomic ratio of 0.5 or less. If the atomicratio of Pt exceeds 0.5, the cost may not be reduced.

Each of Fe (iron), Mn (manganese), and Al (aluminum) represented by B iscontained on the B site alone or in combination in an arbitrary atomicratio so that the total amount of B is the balance of Pt and thetransition element represented by B′ excluding Pt, Fe, Mn, Co, and therare-earth elements.

On the B site, y and z preferably satisfy the following relation:0<y+z≦0.5. Namely, the total atomic ratio of Pt and the transitionelement represented by B′ excluding Pt, Fe, Mn, Co, and the rare-earthelements is 0.5 or less and preferably 0.4 or less. Such an atomic ratiofurther stabilizes the perovskite structure.

In the composite oxide of the present invention having a perovskitestructure, it is preferred that x and z satisfy the relation: x=z,provided that x and z satisfy the relation: 2x=z when A′ is Ag. Namely,the atomic ratio of A′ is preferably equal to that of Pt when A′ is analkaline earth metal, and it is preferably half that of Pt when A′ isAg. By satisfying these conditions, Pt easily stands at a valence of 4and is further stabilized in the perovskite structure.

The composite oxide of the present invention having a perovskitestructure can be prepared according to any suitable procedure for thepreparation of composite oxides. Examples thereof are coprecipitationprocess, citrate complex process, and alkoxide process.

In the coprecipitation process, an aqueous mixed salt solutioncontaining salts of the above-mentioned elements in the stoichiometricratio is prepared. The aqueous mixed salt solution is coprecipitated bythe addition of a neutralizing agent, and the resulting coprecipitate isdried and subjected. to heat treatment.

Examples of the salts of the elements are inorganic salts such assulfates, nitrates, chlorides, and phosphates; and organic salts such asacetates and oxalates, of which nitrates and acetates are preferred. Theaqueous mixed salt solution can be prepared, for example, by adding thesalts of the elements to water so as to establish the stoichiometricratio and mixing them with stirring.

Then, the aqueous mixed salt solution is coprecipitated by adding theneutralizing agent thereto. The neutralizing agent includes, but is notspecifically limited to, ammonia; organic bases including amines such astriethylamine and pyridine; and inorganic bases such as sodiumhydroxide, potassium hydroxide, potassium carbonate, and ammoniumcarbonate. The neutralizing agent is added dropwise to the aqueous mixedsalt solution so that the solution after the addition of theneutralizing agent has a pH of about 6 to 10. This dropwise additionefficiently coprecipitates the salts of the elements.

The resulting coprecipitate is washed with water according to necessity,dried typically by vacuum drying or forced-air drying, and subjected toheat treatment typically at about 500 to 1000° C., preferably at about600 to 950° C. Thus, the composite oxide is prepared.

In the citrate complex process, for example, an aqueous solutioncontaining citrate and a salt mixture is prepared by mixing citric acidand salts of the elements so as to establish the stoichiometric ratio.The aqueous solution containing citrate and a salt mixture is evaporatedto dryness to form citrate complex of the elements. The resultingcitrate complex is provisionally baked and then subjected to heattreatment.

The same as listed above can be used as the salts of the elementsherein. The aqueous solution containing citrate and a salt mixture canbe prepared by initially preparing the aqueous mixed salt solution bythe above procedure and adding an aqueous solution of citric acid to theaqueous mixed salt solution. The amount of citric acid is preferablyabout 2 to 3 moles per 1 mole of the resulting composite oxide.

Then, the aqueous solution containing citrate and a salt mixture isevaporated to dryness to form a citrate complex of the above-mentionedelements. The evaporation to dryness is carried out at a temperature atwhich the formed citrate complex is not decomposed, for example, at roomtemperature to about 150° C. to remove fluid immediately. The citratecomplex of the elements is thus obtained.

The formed citrate complex is then provisionally baked and thensubjected to heat treatment. The provisional baking may be carried outby heating at 250° C. or higher in vacuum or in an inert atmosphere. Theprovisionally baked substance is then heated, for example, at about 500to 1000° C., and preferably at about 600 to 950° C. Thus, the compositeoxide is prepared.

In the alkoxide process, for example, an alkoxide mixed solutioncontaining alkoxides of the elements, except for noble metals such asPt, Ag, Rh, and Ru, in the stoichiometric ratio is prepared. Thealkoxide mixed solution is precipitated on hydrolysis by adding anaqueous solution containing salts of the noble metals such as Pt, Ag,Rh, and Ru thereto. The resulting precipitate is dried and thensubjected to heat treatment.

Examples of the alkoxides of the individual elements include alcholateseach comprising the element and an alkoxy such as methoxy, ethoxy,propoxy, isopropoxy, or butoxy; and alkoxyalcholates of the individualelements represented by the following general formula (2):E[OCH(R¹)—(CH₂)_(a)—OR²]s  (2)wherein E represents the element; R¹ represents a hydrogen atom or analkyl group having 1 to 4 carbon atoms; R² represents an alkyl grouphaving 1 to 4 carbon atoms; a is an integer of 1 to 3; and s is aninteger of 2 or 3.

More specific examples of the alkoxyalcholates include methoxyethylate,methoxypropylate, methoxybutylate, ethoxyethylate, ethoxypropylate,propoxyethylate, and butoxyethylate.

The alkoxide mixed solution can be prepared, for example, by adding thealkoxides of the individual elements to an organic solvent in suchproportions so as to establish the above-mentioned stoichiometric ratioand mixing them with stirring. The organic solvent is not specificallylimited, as long as it can dissolve the alkoxides of the individualelements. Examples of such organic solvents include aromatichydrocarbons, aliphatic hydrocarbons, alcohols, ketones, and esters.Among them, aromatic hydrocarbons such as benzene, toluene, and xylenesare preferred.

Then, the alkoxide mixed solution is precipitated on hydrolysis byadding an aqueous solution containing salts of the noble metals such asPt, Ag, Rh, and Ru thereto in the above-mentioned stoichiometric ratio.Examples of the aqueous solution containing salts of the noble metalssuch as Pt, Ag, Rh, and Ru include aqueous nitrate solution, aqueouschloride solution, aqueous hexaammine chloride solution, aqueousdinitrodiammine nitrate solution, hexachloro acid hydrate, and potassiumcyanide salt.

The resulting precipitate is dried typically by vacuum drying orforced-air drying and is subjected to heat treatment, for example, atabout 500 to 1000° C., and preferably at about 500 to 850° C. Thus, thecomposite oxide is prepared.

In the alkoxide method, the composite oxide may be alternativelyprepared in the following manner. A solution containing organometallicsalts of the noble metals such as Pt, Ag, Rh, and Ru is added to thealkoxide mixed solution to obtain a homogenous mixed solution. Thehomogenous mixed solution is precipitated on hydrolysis by adding waterthereto. The resulting precipitate is dried and then subjected to heattreatment.

Examples of the organometallic salts of the noble metals such as Pt, Ag,Rh, and Ru are metal chelate complexes of the noble metals such as Pt,Ag, Rh, and Ru, including carboxylic acid salts of the noble metals suchas Pt, Ag, Rh, and Ru formed typically from acetate or propionate; anddiketone complexes of the noble metals such as Pt, Ag, Rh, and Ru formedfrom diketone compounds represented by the following general formula(3):R³COCH₂COR⁴  (3)wherein R³ represents an alkyl group having 1 to 4 carbon atoms, afluoroalkyl group having 1 to 4 carbon atoms or an aryl group; and R⁴represents an alkyl group having 1 to 4 carbon atoms, a fluoroalkylgroup having 1 to 4 carbon atoms, an aryl group, or an alkyloxy grouphaving 1 to 4 carbon atoms.

In above-mentioned general formula (3), examples of the alkyl grouphaving 1 to 4 carbon atoms of R³ and R⁴ include, for example, methyl,ethyl, propyl, isopropyl, butyl, s-butyl, and t-butyl. The fluoroalkylgroups each having 1 to 4 carbon atoms of R³ and R⁴ include, forexample, trifluoromethyl. The aryl groups of R³ and R⁴ include, forexample, phenyl. The alkyloxy group having 1 to 4 carbon atoms of R⁴includes, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy,s-butoxy, and t-butoxy.

Specific examples of the diketone compounds include 2,4-pentanedione,2,4-hexanedione, 2,2-dimethyl-3,5-hexanedione, 1-phenyl-1,3-butanedione,1-trifluoromethyl-1,3-butanedione, hexafluoroacetylacetone,1,3-diphenyl-1,3-propanedione, dipivaloylmethane, methyl acetoacetate,ethyl acetoacetate, and t-butyl acetoacetate.

The solution containing the organometallic salts of the noble metalssuch as Pt, Ag, Rh, and Ru can be prepared, for example, by adding anorganometallic salt of the noble metals such as Pt, Ag, Rh, and Ru to anorganic solvent in such proportions as to establish the above-mentionedstoichiometric ratio, and mixing them with stirring. The organic solventherein can be any of the above-mentioned organic solvents.

The above-prepared solution containing organometallic salts of the noblemetals such as Pt, Ag, Rh, and Ru is added to the alkoxide mixedsolution to obtain a homogenous mixed solution, and the homogenous mixedsolution is precipitated on hydrolysis by adding water thereto.

The resulting precipitate is dried typically by vacuum drying orforced-air drying and is subjected to heat treatment, for example, atabout 500 to 1000° C., and preferably at about 500 to 850° C. Thecomposite oxide is thus prepared.

The thus-prepared composite oxide according to the present invention canbe used as intact as an exhaust gas purifying catalyst but is generallysubjected to a conventional procedure to form an exhaust gas purifyingcatalyst. For example, the composite oxide is supported by a catalystcarrier.

The catalyst carrier is not specifically limited and includes, forexample, known catalyst carriers such as honeycomb monolith carriersmade of cordierite and the like.

The composite oxide may be supported by the catalyst carrier, forexample, by adding water to the composite oxide thus obtained to form aslurry, applying the slurry to the catalyst carrier, drying andsubjecting the applied slurry to heat treatment at about 300 to 800° C.,and preferably at about 300 to 600° C.

The resulting exhaust gas purifying catalyst of the present inventioncontaining the composite oxide can stabilize Pt in the perovskitestructure and allows Pt to be finely and highly dispersed in thecomposite oxide and maintains its high catalytic activity even inlong-term use. This is because of self-regenerative function, in whichsolution in an oxidative atmosphere and deposition in a reducingatmosphere are repeated. The self-regenerative function of Pt withrespect to the perovskite structure due to solution and deposition inoxidative-reducing atmospheres can achieve high catalytic activity evenin a small amount of Pt.

As a result, the exhaust gas purifying catalyst of the present inventioncontaining the composite oxide can maintain the catalytic activity of Ptat a high level over a long time and can achieve satisfactory emissioncontrol performance. The catalyst can be suitably used as an exhaust gaspurifying catalyst for automobiles.

EXAMPLES

The present invention will be illustrated in further detail withreference to several examples and comparative examples below, which arenever intended to limit the scope of the invention.

Example 1

Initially, an alkoxide mixed solution was prepared by charging 36.6 g(0.090 mol) of lanthanum ethoxyethylate [La(OC₂H₄OEt)₃], 2.7 g (0.010mol) of strontium ethoxyethylate [Sr(OC₂H₄OEt)₂], 17.4 g (0.054 mol) ofiron ethoxyethylate [Fe(OC₂H₄OEt)₃], and 8.4 g (0.036 mol) of manganeseethoxyethylate [Mn(OC₂H₄OEt)₂] in a 500-mL round-bottomed flask anddissolving them in 200 mL of toluene added thereto with stirring.Separately, 3.93 g (0.010 mol) of platinum acetylacetonate[Pt(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of toluene, and theresulting solution was added to the alkoxide mixed solution in theround-bottomed flask to obtain a homogenous mixed solution containingLaSrFeMnPt.

Next, 200 mL of deionized water was added dropwise to the round-bottomedflask over about fifteen minutes to form a viscous brown precipitate onhydrolysis.

After stirring at room temperature for two hours, toluene and water weredistilled off under reduced pressure to obtain a precursor of theLaSrFeMnPt composite oxide. The precursor was placed on a petri dish,subjected to forced-air drying at 60° C. for twenty-four hours,subjected to heat treatment at 600° C. in the atmosphere for one hourusing an electric furnace to obtain a blackish brown powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.90)Sr_(0.10)Fe_(0.54)Mn_(0.36)Pt_(0.10)O₃. The powder was found tohave a specific surface area of 27 m²/g and a Pt content in thecomposite oxide of 7.77% by mass.

Example 2

Initially, an alkoxide mixed solution was prepared by charging 34.6 g(0.095 mol) of lanthanum methoxyethylate [La(OC₂H₄OMe)₃], 20.2 g (0.080mol) of aluminum methoxyethylate [Al(OC₂H₄OMe)₃], and 2.0 g (0.010 mol)of manganese methoxyethylate [Mn(OC₂H₄OMe)₂] in a 500-mL round-bottomedflask and dissolving them in 200 mL of toluene added thereto withstirring. Separately, 1.04 g (0.005 mol) of silver acetylacetonate[Ag(CH₃COCHCOCH₃)], 3.14 g (0.008 mol) of platinum acetylacetonate[Pt(CH₃COCHCOCH₃)₂], and 0.80 g (0.002 mol) of ruthenium acetylacetonate[Ru(CH₃COCHCOCH₃)₃] were dissolved in 200 mL of toluene. The resultingsolution was added to the alkoxide mixed solution in the round-bottomedflask to obtain a homogenous mixed solution containing LaAgAlMnPtRu.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 800° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.95)Ag_(0.05)Al_(0.80)Mn_(0.10)Pt_(0.08)Ru_(0.02)O₃. The powder wasfound to have a specific surface area of 19 m²/g, and, in the compositeoxide, a Ag content of 2.34% by mass, a Pt content of 6.78% by mass, anda Ru content of 0.88% by mass.

Example 3

Initially, an alkoxide mixed solution was prepared by charging 32.9 g(0.080 mol) of neodymium methoxypropylate [Nd(OCHMeCH₂OMe)₃], 3.2 g(0.001 mol) of barium methoxypropylate [Ba(OCHMeCH₂OMe)₂], 2.0 g (0.010mol) of magnesium methoxypropylate [Mg(OCHMeCH₂OMe)₂], and 25.0 g (0.085mol) of aluminum methoxypropylate [Al(OCHMeCH₂OMe)₃] in a 500-mLround-bottomed flask and dissolving them in 200 mL of xylene addedthereto with stirring. Separately, 3.93 g (0.010 mol) of platinumacetylacetonate [Pt(CH₃COCHCOCH₃)₂] and 2.00 g (0.005 mol) of rhodiumacetylacetonate [Rh(CH₃COCHCOCH₃)₃] were dissolved in 200 mL of xylene.The resulting solution was added to the alkoxide mixed solution in theround-bottomed flask to obtain a homogenous mixed solution containingNdBaMgAlPtRh.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 600° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofNd_(0.80)Ba_(0.10)Mg_(0.10)Al_(0.85)Pt_(0.10)Rh_(0.05)O₃. The powder wasfound to have a specific surface area of 29 m²/g, and, in the compositeoxide, a Pt content of 8.40% by mass and a Rh content of 2.21% by mass.

Example 4

Initially, an alkoxide mixed solution was prepared by charging 36.6 g(0.090 mol) of lanthanum ethoxyethylate [La(OC₂H₄OEt)₃], 2.2 g (0.010mol) of calcium ethoxyethylate [Ca(OC₂H₄OEt)₂], and 29.0 g (0.090 mol)of iron ethoxyethylate [Fe(OC₂H₄OEt)₃] in a 500-mL round-bottomed flaskand dissolving them in 200 mL of toluene with stirring.

Next, 22.9 g (corresponding to 1.95 g (0.010 mol) of Pt) ofdinitrodiammine platinum nitrate solution having a Pt content of 8.50%by mass was diluted with 200 mL of deionized water, and the resultingsolution was added dropwise to the alkoxide mixed solution in theround-bottomed flask over about fifteen minutes to obtain a brownviscous precipitate on hydrolysis.

Subsequently, a blackish brown powder was prepared by the procedure ofExample 1, except for carrying out the heat treatment at 700° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(0.90)Ca_(0.10)Fe_(0.90)Pt_(0.10)O₃. Thepowder was found to have a specific surface area of 19 m²/g and a Ptcontent in the composite oxide of 7.90% by mass.

Example 5

Initially, a homogenous mixed solution was prepared by dissolving andhomogenously mixing 19.1 g (0.050 mol) of yttrium nitrate(Y(NO₃)₃.6H₂O), 7.5 g (0.050 mol) of strontium nitrate (Sr(NO₃)), 20.2 g(0.050 mol) of iron nitrate (Fe(NO₃)₃.9H₂O), and 114.8 g (correspondingto 9.75 g (0.050 mol) of Pt) of dinitrodiammine platinum nitratesolution having a Pt content of 8.50% by mass in 100 mL of pure water.Next, 46.1 g (0.240 mol) of citric acid was dissolved in 100 mL of purewater, and the resulting solution was added to the homogenous mixedsolution to obtain an aqueous solution of citric acid and saltscontaining YSrFePt.

The aqueous solution of citric acid and salts was evaporated to drynesson an oil bath at 60° C. to 80° C. with evacuation using a rotaryevaporator. After passage of about three hours and when the solutioncame into a starch-syrup-like state, the temperature of the oil bath wasgradually raised, followed by drying at 250° C. in vacuum for one hourto obtain a citrate complex.

The above-prepared citrate complex was baked at 300° C. in theatmosphere for three hours and further pulverized in a mortar.Thereafter, it was baked again at 700° C. in the atmosphere for threehours to obtain a powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of Y_(0.50)Sr_(0.50)Fe_(0.50)Pt_(0.50)O₃. Thepowder was found to have a specific surface area of 22.5 m²/g and a Ptcontent in the composite oxide of 37.27% by mass.

Example 6

Initially, an aqueous mixed salt solution containing LaSrMnPt wasprepared by dissolving and homogenously mixing 39.0 g (0.090 mol) oflanthanum nitrate (La(NO₃)₃.6H₂O), 2.7 g (0.010 mol) of strontiumnitrate (Sr(NO₃)₃), 31.4 g (0.090 mol) of manganese nitrate(Mn(NO₃)₂.6H₂O) and 22.9 (corresponding to 1.95 g (0.010 mol) of Pt) ofdinitrodiammine platinum nitrate solution having a Pt content of 8.50%by mass in 200 mL of ion-exchanged water.

The above-prepared solution was coprecipitated by adding an aqueoussolution of ammonium carbonate as a neutralizing agent dropwise theretoto a pH of 10. Then the coprecipitate was fully stirred, filtrated andwashed with water.

The resulting coprecipitate was dried at 120° C. for twelve hours, wasbaked at 700° C. in the atmosphere for three hours to obtain a powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(0.90)Sr_(0.10)Mn_(0.90)Pt_(0.10)O₃. Thepowder was found to have a specific surface area of 20.1 m²/g and a Ptcontent in the composite oxide of 7.78% by mass.

Comparative Example 1

A total of 20 g of a commercially available γ-Al₂O₃ having a specificsurface area of 180 m²/g was impregnated with Pt using 27.1 g(corresponding to 2.3 g of Pt) of a dinitrodiammine platinum nitratesolution having a Pt content of 8.50% by mass, was subjected toforced-air drying at 60° C. for twenty-four hours and was then subjectedto heat treatment at 500° C. in the atmosphere for one hour using anelectric furnace. The amount of Pt supported by γ-Al₂O₃ was 10.3% bymass.

Comparative Example 2

Initially, a homogenous mixed solution was prepared by dissolving andhomogenously mixing 34.6 g (0.080 mol) of lanthanum nitrate(La(NO₃)₃.6H₂O), 3.83 g (0.010 mol) of yttrium nitrate (Y(NO₃)₃.6H₂O),2.74 g (0.010 mol) of strontium nitrate (Sr(NO₃)₃), 21.3 g (0.090 mol)of cobalt nitrate (Co(NO₃)₂.3H₂O), and 22.9 (corresponding to 1.95 g(0.010 mol) of Pt) of a dinitrodiammine platinum nitrate solution havinga Pt content of 8.50% by mass in 100 mL of pure water. Then, 46.1 g(0.240 mol) of citric acid was dissolved in pure water, and theresulting solution was added to the homogenous mixed solution to obtainan aqueous solution of citric acid and salts containing LaYSrCoPt.

Subsequently, a powder was obtained by the procedure of Example 5.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.80)Y_(0.10)Sr_(0.10)Co_(0.90)Pt_(0.10)O₃. The powder was found tohave a specific surface area of 24.6 m²/g and a Pt content in thecomposite oxide of 7.82% by mass.

Comparative Example 3

An aqueous mixed salt solution containing LaSrNiCoPt was prepared bydissolving and homogenously mixing 39.0 g (0.090 mol) of lanthanumnitrate (La(NO₃)₃.6H₂O), 1.35 g (0.005 mol) of strontium nitrate(Sr(NO₃)₃), 1.45 g (0.005 mol) of nickel nitrate (Ni(NO₃)₂.6H₂O), 21.3 g(0.090 mol) of cobalt nitrate (Co(NO₃)₂.3H₂O), and 22.9 (correspondingto 1.95 g (0.010 mol) of Pt) of a dinitrodiammine platinum nitratesolution having a Pt content of 8.50% by mass in 200 mL of ion-exchangedwater.

Subsequently, a powder was prepared by the procedure of Example 6.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(0.90)Sr0.05Ni_(0.05)Co_(0.90)Pt_(0.10)O₃.The powder was found to have a specific surface area of 19.6 m²/g and aPt content in the composite oxide of 7.71% by mass.

Test Example 1

-   1) Coating to catalyst carrier

A total of 120 mL of deionized water was mixed with 20 g of the powdersprepared according to Examples 1 to 6 and Comparative Examples 1 to 3and 100 g of a powdery composite oxide having a composition ofCe_(0.6)Zr_(0.3)Y_(0.1)O_(0.95), followed by addition of 21.1 g ofzirconia sol (NZS-30B, a product of Nissan Chemical Industries, Ltd.; asolid content of 30% by mass) to obtain a slurry. The slurry was appliedby coating to a catalyst carrier comprising a cordierite honeycombhaving a diameter of 80 mm, a length of 95 mm, and a grating density of400 cells/(0.025 m)².

After coating, excess slurry was removed by air blow so as to set thecoating amount of the powder at 126 g per 1 L of the catalyst carrier(60 g per one catalyst carrier). After forced-air drying at 120° C. fortwelve hours, the work was baked at 600° C. in the atmosphere for threehours to obtain monolith catalysts containing the powders according toExamples 1 to 6 and Comparative Examples 1 to 3, respectively.

-   2) Endurance test

The above-prepared monolith catalysts were connected to both banks of aV type eight cylinder engine having a displacement of 4 liters and weresubjected to an endurance test at a highest temperature in the catalystbed of 1050° C. with a single cycle of 30 seconds repeated for a totalof 60 hours.

One cycle of the endurance test was set as follows. Specifically, fromSecond 0 to Second 5 (a period of 5 seconds), the operation was carriedout at a theoretical fuel-air ratio (λ=1). From Second 5 to Second 28 (aperiod of 23 seconds), an excessive amount of fuel was fed to the bed(λ=0.89). From Second 7 to Second 30 (a period of 23 seconds) laggingtwo seconds from the above, high-pressure secondary air was introducedupstream of the catalysts. From Second 7 to Second 28 (a period of 21seconds), a slightly excessive amount of air was fed (λ=1.02) to causethe excessive fuel to burn in the interior of the bed, so as to raisethe temperature of the catalyst bed to 1050° C. From Second 28 to Second30 (a period of 2 seconds), the interior of the bed was returned to thetheoretical fuel-air ratio (λ=1) and the secondary air was kept to befed to achieve a high-temperature oxidative atmosphere in which the airis in large excess (λ=1.25).

-   3) Activity determination

Using an in-line four-cylinder engine having a displacement of 1.5liters, an oscillation (amplitude) of Δλ=±3.4% (ΔA/F=±0.5 A/F) of whichthe center was set in the theoretical fuel-air ratio (λ=1) was appliedto the monolith catalysts at a frequency of 1 Hz. The purification ratesof CO, HC, and NOx of the monolith catalysts before and after thisendurance test were measured. The results are shown in Table 1. In themeasurement, the temperature of the upstream (inlet gas) of the monolithcatalysts was kept at 460° C., and the flow rate was set at a spacevelocity (SV) of 50000 per hour. Table 1 also shows the noble metalcontent (g) per 1 liter of each of the monolith catalysts. TABLE 1 Noblemetal Purification rate before Purification rate after content endurancetest (%) endurance test (%) Catalyst Composition (g/L catalyst) CO HCNOx Co HC NOx Example 1 La_(0.90)Sr_(0.10)Fe_(0.54)Mn_(0.36)Pt_(0.10)O₃Pt: 1.55 94.3 96.8 93.8 85.7 88.5 85.1 Example 2La_(0.95)Ag_(0.05)Al_(0.80)Mn_(0.10)Pt_(0.08)Ru_(0.02)O₃ Pt: 1.35 96.693.4 97.2 87.6 82.0 81.7 Ru: 1.76 Ag: 0.47 Example 3Nd_(0.80)Ba_(0.10)Mg_(0.10)Al_(0.85)Pt_(0.10)Rh_(0.05)O₃ Pt: 1.68 99.3100 100 92.0 89.7 92.5 Rh: 0.441 Example 4La_(0.90)Ca_(0.10)Fe_(0.90)Pt_(0.10)O₃ Pt: 1.58 95.3 98.7 95.5 81.9 80.682.6 Example 5 Y_(0.50)Sr_(0.50)Fe_(0.50)Pt_(0.50)O₃ Pt: 7.45 98.6 99.098.2 94.1 93.3 95.0 Example 6 La_(0.90)Sr_(0.10)Mn_(0.90)Pt_(0.10)O₃ Pt:1.56 93.2 95.1 92.4 84.0 86.4 83.2 Comparative Pt-supporting/γ-Al₂O₃ Pt:2.06 98.8 92.1 87.0 66.7 60.8 27.1 Example 1 comparativeLa_(0.80)Y_(0.10)Sr_(0.10)Co_(0.90)Pt_(0.10)O₃ Pt: 1.56 94.3 90.0 96.175.5 68.3 65.2 Example 2 ComparativeLa_(0.90)Sr_(0.05)Ni_(0.05)Co_(0.90)Pt_(0.10)O₃ Pt: 1.54 92.0 91.1 95.372.1 69.5 64.0 Example 3

Table 1 shows that the monolith catalysts comprising the powdersaccording to Comparative Examples 1 to 3 exhibit markedly decreasedpurification rates after the endurance test, and that, in contrast, themonolith catalysts comprising the powders according to Examples 1 to 6maintain their high activities even after the endurance test.

While the illustrative embodiments and examples of the present inventionare provided in the above description, such is for illustrative purposeonly and it is not to be construed restrictively. Modification andvariation of the present invention which will be obvious to thoseskilled in the art is to be covered in the following claims.

Industrial Applicability

The exhaust gas purifying catalyst of the present invention can maintainthe catalytic activity of Pt at a high level over a long time, canachieve satisfactory emission control performance and are advantageouslyusable as an exhaust gas purifying catalyst for automobiles.

1. An exhaust gas purifying catalyst comprising a composite oxide havinga perovskite structure represented by the general formula (1):A_(1-x)A′_(x)B_(1-y-z)B′_(y)Pt_(z)O₃  (1)wherein A represents at leastone element selected from rare-earth elements which essentially includeone or more rare-earth elements each having a valence of 3 as the onlyvalence; A′ represents at least one element selected from alkaline earthmetals and Ag; B represents at least one element selected from Fe, Mn,and Al; B′ represents at least one element selected from transitionelements excluding Pt, Fe, Mn, Co, and the rare-earth elements; x is anatomic ratio satisfying the following relation: 0<x≦0.5; y is an atomicratio satisfying the following relation: 0≦y<0.5; and z is an atomicratio satisfying the following relation: 0<z≦0.5.
 2. The exhaust gaspurifying catalyst according to claim 1, wherein, in the general formula(1), A represents at least one element selected from La, Nd, and Y; A′represents at least one element selected from Mg, Ca, Sr, Ba, and Ag;and B′ represents at least one element selected from Rh and Ru.
 3. Theexhaust gas purifying catalyst according to claim 1, wherein y and z inthe general formula (1) satisfy the following relation: 0<y+z≦0.5. 4.The exhaust gas purifying catalyst according to claim 1, wherein x and zin the general formula (1) satisfy the following relation: x=z, providedthat x and z satisfy the following condition: 2x=z when A′ is Ag.
 5. Acatalyst composition comprising a composite oxide having a perovskitestructure represented by the general formula (1):A_(1-x)A′_(x)B_(1-y-z)B′_(y)Pt_(z)O₃  (1)wherein A represents at leastone element selected from rare-earth elements which essentially includeone or more rare-earth elements each having a valence of 3 as the onlyvalence; A′ represents at least one element selected from alkaline earthmetals and Ag; B represents at least one element selected from Fe, Mn,and Al; B′ represents at least one element selected from transitionelements excluding Pt, Fe, Mn, Co, and the rare-earth elements; x is anatomic ratio satisfying the following relation: 0<x≦0.5; y is an atomicratio satisfying the following relation: 0≦y<0.5; and z is an atomicratio satisfying the following relation: 0<z≦0.5.